pthread_setname_np shim should return and int on macOS, but returns void.

[thomcc]

pthread_setname_np on macOS returns an integer success code: https://github.com/apple-oss-distributions/libpthread/blob/cf9e1c7e611440e511af230905be2cfefc5c6121/include/pthread/pthread.h#L509-L510

Unfortunately, the miri shim for it returns void:

miri/src/shims/unix/macos/foreign_items.rs

Lines 176 to 185 in c3d67d4

	"pthread_setname_np" => {
	let [name] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
	let thread = this.pthread_self()?;
	let max_len = this.eval_libc("MAXTHREADNAMESIZE")?.to_machine_usize(this)?;
	this.pthread_setname_np(
	thread,
	this.read_scalar(name)?,
	max_len.try_into().unwrap(),
	)?;
	}

This probably happened because the manpage on apple tragically has the same mistake: https://github.com/apple-oss-distributions/libpthread/blob/cf9e1c7e611440e511af230905be2cfefc5c6121/man/pthread_setname_np.3#L35

But the function truly does return int: https://github.com/apple-oss-distributions/libpthread/blob/cf9e1c7e611440e511af230905be2cfefc5c6121/src/pthread.c#L1140-L1163

This leads to false positives for UB in libstd's tests: https://github.com/rust-lang/miri-test-libstd/actions/runs/3342425264/jobs/5534636230#step:5:723

[thomcc]

The fix for this is probably

diff --git a/src/shims/unix/macos/foreign_items.rs b/src/shims/unix/macos/foreign_items.rs
index 371f56ca..221dc396 100644
--- a/src/shims/unix/macos/foreign_items.rs
+++ b/src/shims/unix/macos/foreign_items.rs
@@ -177,11 +177,14 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let [name] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
                 let thread = this.pthread_self()?;
                 let max_len = this.eval_libc("MAXTHREADNAMESIZE")?.to_machine_usize(this)?;
-                this.pthread_setname_np(
+                let res = this.pthread_setname_np(
                     thread,
                     this.read_scalar(name)?,
                     max_len.try_into().unwrap(),
                 )?;
+                // Contrary to the manpage, `pthread_setname_np` on macOS still
+                // returns an integer indicating success.
+                this.write_scalar(res, dest)?;
             }
             "pthread_getname_np" => {
                 let [thread, name, len] =

but I can't be bothered to check (my miri setup lapsed and this deserves a test anyway).

[JakobDegen]

This would be why we have autogenerated docs 🙃

[thomcc]

I'll file a radar this weekend to ask them to fix it, but I wouldn't hold my breath.